1. Field of the Invention
This invention relates generally to switching converters, and more particularly to circuits for protecting the low side switching element of such a converter from excessive negative current.
2. Description of the Related Art
A typical switching power converter has high and low side switching elements which are connected to an output inductor; the switching elements are switched in complementary fashion to produce an output voltage.
Under certain conditions—for example, when the load being driven becomes light or transitions from a heavy load to a light load, or if a fault condition causes the output voltage to be higher than the nominal output voltage, or during a dynamic voltage scaling (DVS) transition from a voltage to a lower voltage (e.g., if the converter is programmed to change Vout from 3.3V to 1.5V)—the current in the output inductor can become negative. If the current becomes excessively negative, the low side switching element may be damaged. Therefore, it may be necessary to protect the low side switching element by limiting the negative current to which the switching element is subjected. The low side switching element can be, for example, a FET, BJT, diode, etc.; for simplicity, the discussion herein assumes that the low side switching element is a FET.
One way of limiting the negative current is to generate a reference voltage Vref, and then to compare the Vds of the low side FET with Vref. If the on-resistance (Ron) of the low side FET is known, a current limit for the FET can be established as Vref/Ron; then comparing Vds to Vref determines whether the limit has been exceeded. However, if the Ron of the low side FET is unknown, because the FET is selected by the end user, for example, it is not possible to establish an accurate limit for the low side FET in this way.
Another approach is described in U.S. Pat. No. 7,928,713 to Nguyen, in which the negative current is limited by forcing the low side MOSFET to operate in its saturation region; however, this may result in an unacceptably high level of power loss.